1. Field of the Invention
The invention relates generally to ignition devices for internal combustion engines, such as spark plugs, and more particularly to the electrodes therefore.
2. Description of the Prior Art
Internal combustion engines include ignition devices, such as spark ignition devices or spark plugs that extend to the combustion chamber and produce a spark to ignite a mixture of air and fuel. Recent advancements in engine technology are resulting in higher engine operating temperatures to achieve improved engine efficiency. These higher operating temperatures, however, are pushing electrodes of the spark plugs to the very limits of their material capabilities. Presently, Ni-based alloys, including nickel-chromium-iron alloys specified under UNS N06600, such as those sold under the trade names Inconel 600®, Nicrofer 7615®, and Ferrochronin 600®, are typically used as spark plug electrode materials.
As is well known, the resistance to high temperature oxidation of these Ni-based nickel-chromium-iron alloys decreases as their operating temperature increases. Since combustion environments are highly oxidizing, corrosive wear including deformation and fracture caused by high temperature oxidation and sulfidation can result and is particularly exacerbated at the highest operating temperatures. At the upper limits of operating temperature (e.g., 1400° F.), tensile, creep rupture and fatigue strength also have been observed to decrease significantly which can result in deformation, cracking and fracture of the electrodes. Depending on the electrode design, specific operating conditions and other factors, these high temperature phenomena may contribute individually and collectively to undesirable corrosion and erosion of the electrode and diminished performance of the ignition device and associated engine, especially in high performance engines, such as those used in automobile racing.
High temperature firing tips have been employed in conjunction with the electrode materials described. These firing tips have been manufactured from a number of platinum group metals and metal alloys, such as platinum, iridium, rhodium, palladium, ruthenium and rhenium, as pure metals and together with themselves and various other alloy constituents, such as various rare earth elements, in various alloy combinations; gold and gold alloys; tungsten and tungsten alloys and the like. These high temperature firing tips have been attached to a body portion of the electrode materials described above, both center and ground electrodes, in various tip configurations using a wide variety of attachment and joining techniques, including resistance welding, laser welding, mechanical joining and the like, both separately and in various combinations.
Notwithstanding the electrode performance improvements attainable through the use of high temperature firing tips, there remain various aspects of these materials which limit their application and use in ignition device configurations and applications, for example susceptibility to other and new high temperature oxidation, erosion and corrosion mechanisms, such as those associated with small amounts of calcium and phosphorus, thermal expansion mismatch with various center and ground electrode materials and other aspects, such as the high cost of these materials, which serve to limit their usefulness in various ignition applications.